Make placeholder text to fit in visible area by non-scrollable wrapped label. Placeholder style is tuned to attenuate it comparing to header.
I'm not yet sure, what styles combination is best. When it's in treeview, it also has monospace style among all differences. DId not try stack approach yet.
There's look with my gtk style:  
You can view, comment on, or merge this pull request online at:
https://github.com/geany/geany-plugins/pull/1150
-- Commit Summary --
* Better placeholder text implementation
-- File Changes --
M workbench/src/plugin_main.c (2) M workbench/src/sidebar.c (27)
-- Patch Links --
https://github.com/geany/geany-plugins/pull/1150.patch https://github.com/geany/geany-plugins/pull/1150.diff